If you play "Prismatic Omen" and then you play Valakut, valakut will never trigger when other land comes into the battlefield under your control. However, if you play first valakut, it triggers perfectly.

I tried the same with Emeria, the sky ruin, and it works perfectly always. same with bojuka bog. So, maybe prismatic omen only nullify a trigger when something come into battefield, and not a CIP or trigger during upkeep.